Soft tissue paper is generally preferred for disposable paper towels, and facial and toilet tissues. However, known methods and means for enhancing softness of tissue paper generally adversely affect tensile strength. Tissue paper product design is, therefore, generally, an exercise in balancing softness against tensile strength. Both mechanical and chemical means have been introduced in the pursuit of making soft tissue paper: tissue paper which is perceived by users, through their tactile sense, to be soft. Such tactile perceivable softness may be characterized by, but not limited to, friction, flexibility, and smoothness; and subjective descriptors such as feeling like silk or flannel. The present invention pertains to a process for improving the tactile perceivable softness of tissue paper--in particular high bulk, creped tissue paper--through the incorporation of chemical additives: in particular, polysiloxane materials which impart a silky or flannel-like feel to the tissue paper without rendering it greasy or oily to the tactile sense of users of products comprising such tissue paper. Additionally, surfactant material may be added to further enhance softness and/or surface smoothness and/or to at least partially offset any reduction in wettability caused by the polysiloxane; and binder material such as starch may be added to at least partially offset reductions in strength and or increasing in linting proclivity that results from the polysiloxane and, if used, the surfactant additive.
Representative high bulk, creped tissue papers which are quite soft by contemporary standards, and which are susceptible to softness enhancement through the present invention are disclosed in the following U.S. patents: U.S. Pat. No. 3,301,746 which issued Jan. 31, 1967, to Lawrence H. Sanford and James B. Sisson; U.S. Pat. No. 3,974,025 which issued Aug. 10, 1976, to Peter G. Ayers; U.S. Pat. No. 3,994,771 which issued Nov. 30, 1976, to George Morgan, Jr. and Thomas F. Rich; U.S. Pat. No. 4,191,609 which issued Mar. 4, 1980, to Paul D. Trokhan; and U.S. Pat. No. 4,637,859 which issued Jan. 20, 1987, to Paul D. Trokhan. Each of these papers is characterized by a pattern of dense areas: areas more dense than their respective remainders, such dense areas resulting from being compacted during papermaking as by the crossover knuckles of imprinting carrier fabrics. Other high bulk, soft tissue papers are disclosed in U.S. Pat. No. 4,300,981 which issued Nov. 17, 1981, to Jerry E. Carstens; and U.S. Pat. No. 4,440,597 which issued Apr. 3, 1984, to Edward R. Wells and Thomas A. Hensler. Additionally, achieving high bulk tissue paper through the avoidance of overall compaction prior to final drying is disclosed in U.S. Pat. No. 3,821,068 which issued Jun. 28, 1974, to D. L. Shaw; and avoidance of overall compaction in combination with the use of debonders and elastomeric bonders in the papermaking furnish is disclosed in U.S. Pat. No. 3,812,000 which issued May 21, 1974, to J. L. Salvucci, Jr.
Chemical debonders such as those contemplated by Salvucci, referred to above, and their operative theory are disclosed in such representative U.S. patents as U.S. Pat. No. 3,755,220 which issued Aug. 28, 1973, to Friemark et al.; U.S. Pat. No. 3,844,880 which issued Oct. 29, 1974, to Meisel et al.; and U.S. Pat. No. 4,158,594 which issued Jan. 19, 1979, to Becker et al.
Tissue paper has also been treated with cationic surfactants, as well as noncationic surfactants to enhance softness. See, for example, U.S. Pat. No. 4,959,125 which issued Sep. 25, 1990, to Spendel; and U.S. Pat. No. 4,940,513 which issued Jul. 10, 1990, to Spendel, that disclose processes for enhancing the softness of tissue paper by treating it with noncationic, preferably nonionic, surfactants.
It has been found that the softness of tissue paper, in particular, high bulk pattern densified tissue papers, can be improved by treatment with various agents such as vegetable, animal or synthetic oils, and especially polysiloxane materials typically referred to as silicone oils. See, for example, U.S. Pat. No. 5,059,282 which issued Oct. 22, 1991, to Ampulski et al. The Ampulski patent discloses a process for adding a polysiloxane compound to a wet tissue web (preferably at a fiber consistency of between about 20% and about 35%). These polysiloxane compounds impart a silky, soft feeling to the tissue paper. However, addition of the polysiloxane to the tissue web before the web is dried and creped, in accordance with the process disclosed in U.S. Pat. No. '282, can result in interference with the coating on the Yankee dryer and also cause skip crepe and a loss in sheet control. Importantly, these problems are eliminated by the process of the present invention wherein the polysiloxane is added to the tissue sheet after the sheet leaves the Yankee dryer.
U.S. Pat. No. 5,246,546 which issued Sep. 21, 1993 to Ampulski, and incorporated herein by reference discloses an improved process for making soft tissue paper by the application of expensive functional polydimethylpolysiloxane compounds to a dry tissue paper web. Unfortunately, functional polydimethylpolysiloxane compounds are quite expensive, and it is of great economic importance to apply only the minimal quantity required to achieve the desired softness benefit. Surprisingly, Applicant has found that when the functional polydimethylpolysiloxane compounds are first diluted with a miscible, nonvolatile inexpensive solvent such as a nonfunctional polysiloxane compound or a mineral oil, equivalent softness benefits can be obtained with a fraction of the costly functional polydimethylpolysiloxane compounds. It is believed that the addition of the nonfunctional polysiloxane allows the active functional polydimethylpolysiloxane compounds to spread more uniformly on the tissue sheet at lower concentration levels. Importantly, the silicone blends described in the present invention offer substantial cost savings over the higher concentration functional polydimethylpolysiloxane formulations disclosed in U.S. Pat. No. '546.
Additionally, a well known mechanical method of increasing tensile strength of paper made from cellulosic pulp is by mechanically refining the pulp prior to papermaking. In general, greater refining results in greater tensile strength. However, consistent with the foregoing discussion of tissue tensile strength and softness, increased mechanical refining of cellulosic pulp negatively impacts tissue paper softness, all other aspects of the papermaking furnish and process being unchanged. However, through the use of the present invention, tensile strength can be increased without negatively impacting softness; or, alternatively, softness can be improved without negatively impacting tensile strength.
It is an object of this invention to provide a process for preparing tissue paper which has an enhanced tactile sense of softness.
It is another object of this invention to provide a process for preparing tissue paper which has a silky, flannel-like feel.
It is another object of this invention to provide a process for preparing tissue paper which has increased tactile softness at a particular level of tensile strength relative to tissue paper which has been softened by conventional techniques.
It is a another object to provide a process for preparing a soft tissue paper by applying a functional-polysiloxane compound to a dry tissue web from a thin film.
It is a further object to provide a process for softening tissue paper that only requires very low levels of expensive functional-polysiloxanes compounds.
These and other objects are obtained using the present invention, as will be seen from the following disclosure.